The fields of industrial application of the ultrafine oxides have been expanding considerably in recent years. For instance, ultrafine titanium oxide is being extensively studied as an ultraviolet-shielding agent, an additive for a silicone rubber, and a photocatalyst. In particular, the application for cosmetics attracts special attention due to the ultraviolet shielding effect of ultrafine titanium oxide, and, in light of the photocatalytic properties of the ultrafine titanium oxide, some attention is also being paid to the application for prevention of fouling, sterilization, and deodorizing. Such applications are supported by the advantages of ultrafine titanium oxide in terms of safety, processability, functional characteristics, and durability. The ultrafine particles have not been exactly defined, but are generally regarded as fine particles with a primary particle diameter of about 0.1 μm or less.
The specific functions of titanium oxide, that is, scattering and absorption of ultraviolet light, are noteworthy. It is more noticeable that ultrafine particles of titanium oxide are favorably provided with the above-mentioned two functions in combination. For instance, ultrafine titanium oxide with a primary particle diameter of about 80 nm can work to effectively scatter ultraviolet light. In addition, it is known that such ultrafine particles of titanium oxide can effectively absorb ultraviolet light with a wavelength of about 400 nm or less and be excited to generate electrons and/or holes in the portion adjacent to the surface of the particle, thereby exhibiting such photocatalytic performance as to carry out the prevention of fouling, sterilization, and deodorizing, as mentioned above.
However, when titanium oxide having such functions is used for cosmetic applications in practice, there is the possibility that the titanium oxide works improperly unless subjected to a surface treatment (coating). This is because the electrons and holes caused by photo-excitation generate various radicals when allowed to react with oxygen and water in the air, so that they work to decompose organic materials in the air.
Titanium oxide is also used as a high-performance dielectric material. For example, titanium oxide is subjected to a solid phase reaction with barium carbonate at 1,200° C. in accordance with the following reaction formula, thereby providing barium titanate serving as a dielectric material.BaCO3+TiO2→BaTiO3+CO2
In this case, barium carbonate decomposes at around 700° C. to generate BaO with high tendency of ionization, which is diffused into TiO2 particles with covalent bonding characteristics to form a solid solution, thereby producing barium titanate. The particle size of the barium titanate is determined by the crystalline size of the TiO2 in the course of the reaction. Therefore, the crystallinity and the particle size of the titanium oxide serving as the raw material become significant. To cope with the requirement for a small-size ceramic condenser with a high dielectric constant, there is an increasing demand for ultrafine particles of barium titanate, and consequently, for ultrafine particles of titanium oxide as a raw material.
However, the growth of titanium oxide particles with a particle size of 0.1 μm or less is striking at the above-mentioned reaction temperature of about 700° C., so that there is the problem that such titanium oxide particles cannot contribute to the provision of ultrafine particles of barium titanate. Ultrafine particles of titanium oxide for achieving the above-mentioned object is desired.
As an example of a method for producing fine particles of a composite containing titanium oxide, a production process is known for finely-divided particles of silica—titania composite material, that is, a production process of allowing a mixture of gaseous halogenated silicon and gaseous halogenated titanium to react with oxidizing gas containing an oxygen at 900° C. or more (Japanese Laid-Open Patent Application No. 50-115190). According to this method, the mixture of gases serving as the raw material is subjected to a reaction under conditions of a high temperature of 900° C. or more without preheating. The resultant composite particles have such a structure that crystalline TiO2 particles are always deposited on the surface of the composite particles.
Japanese Patent No. 2503370 (European Patent No. 595078) discloses that a mixed oxide of titanium oxide, aluminum oxide, and silicon oxide can be produced by flame hydrolysis (at a reaction temperature of 1000 to 3000° C.) using chlorides as raw materials. The flame hydrolysis produces a mixed oxide of Al2O3 and TiO2, or a mixed oxide of SiO2 and TiO2. Similarly, Japanese Patent No. 2533067 (European Patent No. 585544) discloses manufacture of a mixed oxide of aluminum oxide and silicon oxide by flame hydrolysis.
As previously mentioned, the production process for a metal oxide by a vapor phase method, or the production process for a metal oxide or mixed metal oxide by flame hydrolysis is conventionally known. However, the growing mechanism of the product particles that is, in general, seriously influenced by the reaction temperature, the gas flow velocity, the cooling rate, or the like, has not been sufficiently clarified.